1. Technical Field
The present invention relates to wireless communication and, in particular, to a method of generating random access preambles in a wireless communication system.
2. Related Art
The 3GPP (3rd Generation Partnership Project) mobile communication system based on WCDMA (Wideband Code Division Multiple Access) radio access technologies is widely deployed all over the world. An HSDPA (High Speed Downlink Packet Access), which could be defined as the first evaluation phase of the WCDMA, provides radio access technologies that are highly competitive in the mid-term future. However, because radio access technologies are being constantly advanced to meet the increasing demands and expectations of users and providers, new technological evolution is required in the 3GPP to ensure competitiveness in the future.
One of the systems that are considered to follow the 3rd generation systems is an OFDM (Orthogonal Frequency Division Multiplexing) system that attenuates inter-symbol interference (ISI) with low complexity. In the OFDM, serially inputted data symbols are converted into the N number of parallel data symbols, transmitted on the N number of orthogonal subcarriers. The subcarriers maintain orthogonality in frequency domain. Respective orthogonal channels experience mutually independent frequency selective fading, and when the interval between symbols is long enough, ISI can be canceled. An OFDMA (Orthogonal Frequency Division Multiple Access) refers to a multiple access method using the OFDM as modulation scheme. In the OFDMA, the frequency resources, namely, the subcarriers, are provided to each user. In this case, because each frequency resource is independently provided to a plurality of users, the frequency resources do not overlap with each other. Namely, the frequency resources are allocated to the users exclusively.
In order to transmit or receive a data packet, control information needs to be transmitted. For example, uplink control information includes ACK (Acknowledgement)/NACK(Negative-Acknowledgement) signals indicating successful transmission of downlink data, a CQI (Channel Quality Indicator) indicating quality of a downlink channel, a PMI (Precoding Matrix Index), an RI (Rank Indicator), etc. In addition, a random access preamble needs to be transmitted to perform a random access procedure.
A sequence is commonly used to transmit the uplink control information or the random access preamble. The sequence is transmitted in the form of a spreading code, a user equipment identifier, or a signature via a control channel or a random access channel.
FIG. 1 is an exemplary view showing a method for performing a random access procedure in a WCDMA system. The random access procedure is performed to allow a user equipment to acquire uplink synchronization with a network or acquire uplink radio resources for transmitting uplink data.
Referring to FIG. 1, a user equipment transmits a preamble via a PRACH (Physical Random Access Channel) which is an uplink physical channel. The preamble is transmitted during the access slot of 1.33 ms. The preamble is randomly selected from sixteen preambles.
Upon receiving the preamble from the user equipment, a base station transmits a response via an AICH (Acquisition Indicator Channel) which is a downlink physical channel. The base station transmits an acknowledgement (ACK) or a negative acknowledgement (NACK) to the user equipment via the AICH. If the user equipment receives ACK, the user equipment transmits a message having a length of 10 ms or 20 ms by using an OVSF (Orthogonal Variable Spreading Factor) code corresponding to the preamble. If the user equipment receives NACK, the user equipment transmits the preamble again in a suitable time. If the user equipment fails to receive a response corresponding to the previously transmitted preamble, the user equipment transmits a new preamble with power level higher than that of the previous preamble after a determined access slot.
The user equipment acquire information on sixteen preambles (namely, sequences), and uses one selected from the sixteen preambles as a preamble in the random access procedure. If the base station informs the user equipment of information regarding every available sequence, signaling overhead may be increased. So, generally, the base station previously designates sets of sequences and transfers an index of the sets of sequences to the sixteen preambles. For this purpose, the user equipment and the base station should store the sets of sequences according to the index in their buffer, respectively. This may be burdensome if the number of sequences belonging to the sequence sets is increased or the number of sets of sequences is increased.
In order to enhance performance of data detection in a receiver and increase capability, correlation or CM (Cubic Metric) characteristics of the sequences should be guaranteed to a degree. This means that the sequences belonging to the sequence sets used for the random access procedure should have correlation or CM characteristics guaranteed by more than a certain level. In particular, a sequence used for a high speed environment in which the user equipment is moved by a speed of 30 km/h or faster and a sequence used for a low speed environment need to be separately used in order to guarantee sequence characteristics in consideration of Doppler effect.
A method is sought for guaranteeing the characteristics of sequences used for transmission of the uplink control information with smaller amount of signaling overhead.